In `wave.resource.surface_elevation` use `sum_of_sines` method if input spectrum does not have a zero frequency

[simmsa]

Add auto method to wave.resource.surface_elevation to address #308.

The auto method chooses the most computationally efficient surface elevation computation method based on the input spectrum. auto uses the sum_of_sines method is a zero frequency is not defined in the input spectrum index and uses the ifft for all other cases.